(1) Field of the Invention
The present invention generally relates to a traction control apparatus for an automotive vehicle, and more particularly to a traction control apparatus for controlling a driving force on driving wheels of the vehicle due to an output torque of an engine, so as to prevent the vehicle from undergoing excessive slippage when the vehicle is accelerating.
(2) Description of the Related Art
When an automotive vehicle is accelerating on a snowy road, the driving force transmitted from the engine to the driving wheel of the vehicle often exceeds the maximum friction force between the wheel and the road. Since the maximum friction force between the wheel and the snowy road is relatively low, the vehicle undergoes excessive slippage at the driving wheels. If the driving wheels slip, a side force acting in the direction of vehicle movement and the side force acting in a direction lateral to the vehicle movement are excessively lowered. As a result, the vehicle acceleration is lowered and the vehicle running condition becomes unstable.
In order to eliminate the above problem, there is a known traction control device for automotive vehicles. In the known traction control device, a slip ratio is determined based on a speed difference between the driving wheel speed and the vehicle running speed, and the driving force produced by the engine is controlled such that the slip ratio is adjusted to be a given target slip ratio.
Japanese Laid-Open Patent Publication No. 2-161142 discloses a traction control device of the kind described above. In the traction control device disclosed in the above mentioned publication, a throttle position of a throttle valve provided in an intake passage of an engine is set to a prescribed value for a high road friction when the execution of the traction control process is started, and a feedback control process (for adjusting the throttle position of the sub-throttle valve) as well as a fuel cut process (for cutting the fuel injected to the engine by a fuel injection valve), is started before the throttle position is subsequently changed. The execution of the fuel cut process ends when the driving wheel rotational acceleration is smaller than a prescribed acceleration value.
If the feedback control process is continuously performed during a time period from the start of the traction control process to the end of the fuel cut process as in the above traction control apparatus, the throttle position of the sub-throttle valve is set to an excessively small value due to the execution of the feedback control process. When the traction control process is started at the end of the fuel cut process, the sub-throttle valve is at such a small throttle position, and the vehicle acceleration is lowered and unstable.